linux-kernel - Re: [PATCH v2 16/19] gendwarfksyms: Add support for reserved structure fields

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Message-ID: <CABCJKuer=O3FnLJNGMg2+-HxFJFUrccTuuHt5OiMpRsAJBvBsg@mail.gmail.com>
Date: Thu, 12 Sep 2024 09:06:19 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Petr Pavlu <petr.pavlu@...e.com>
Cc: Miroslav Benes <mbenes@...e.cz>, Masahiro Yamada <masahiroy@...nel.org>, 
	Luis Chamberlain <mcgrof@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Matthew Maurer <mmaurer@...gle.com>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, 
	Neal Gompa <neal@...pa.dev>, Hector Martin <marcan@...can.st>, Janne Grunau <j@...nau.net>, 
	Asahi Linux <asahi@...ts.linux.dev>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 16/19] gendwarfksyms: Add support for reserved
 structure fields

On Wed, Sep 11, 2024 at 4:43 AM Petr Pavlu <petr.pavlu@...e.com> wrote:
>
> On 8/31/24 02:05, Sami Tolvanen wrote:
> > On Fri, Aug 30, 2024 at 9:34 AM Miroslav Benes <mbenes@...e.cz> wrote:
> >>
> >> yes, this is one of the approaches we use in SLES. We add kabi paddings
> >> to some structures in advance (see [1] as a random example) and then use
> >> it later if needed.
> >>
> >> It is not the only approach. Much more often we do not have a padding and
> >> use alignment holes ([5]), addition of a new member to the end of a
> >> structure ([2] or [3]) and such "tricks" ([4] for a newly fully defined
> >> structure).
> >
> > Thanks for bringing this up! Sounds like we're also going to need a
> > way to completely exclude specific fields from the output then. I
> > think we can use a similar union approach, but instead of instructing
> > the tool to use another type, we can just indicate that the field
> > should be skipped. I'll come up with a solution for v3.
>
> It might have been mentioned previously, not sure, but one more case to
> consider is handling of enum declarations. New enumerators can be
> typically added without breaking ABI, e.g. 'enum E { OLD1, OLD2, NEW }'.
> It would be then great to have some ability to hide them from
> gendwarfksyms.
>
> I think neither of the __kabi_reserved or __gendwarfksyms_declonly
> mechanism can currently help with that.

I thought about this a bit and I wonder if we need a separate
mechanism for that, or is it sufficient to just #define any additional
hidden values you want to add instead of including them in the enum?

  enum e {
      A,
      B,
  #define C (B + 1)
  #define D (C + 1)
  };

Do you see any issues with this approach? I think Clang would complain
about this with -Wassign-enum, but I'm not sure if we even enable that
in the kernel, and as long as you don't overflow the underlying type,
which is a requirement for not breaking the ABI anyway, it should be
fine.

Sami